In this paper, we report the effects of O2-plasma treatment on the reliability and electrical properties of indium tin zinc oxide (ITZO) films. Excellent electrical properties, including a saturation mobility (μsat) of ~20.2 cm2/V · s, a threshold voltage (VTH) of ~-6.8 V, a sub-threshold swing (S.S) of ~0.956 V/decade, and an on/off current ratio (ION/OFF) of ~10(5) can be found with a molarity of 0.4 M and ratio of In:Zn:Sn = 2:1:2. Following O2-plasma treatment, it was confirmed that the electrical properties of the ITZO films are improved when compared to the untreated films. The devices showed a decreased S.S of ~0.51 V/decade, while the VTH and ION/OFF tended to increase. To determine the reliability of a-ITZO TFTs, we analyzed the electrical characteristics according to gate bias stress, VG,stress = 10 V for 4000 s. Improved reliability was confirmed when compared with the variation in threshold voltage prior to O2-plasma treatment, most likely stemming from a smooth surface on the active layer as a result of O2-plasma treatment. We were able to obtain a solution a-ITZO film transmittance of 92% in the visible light region (400~700 nm). These results show that a-ITZO TFTs fabricated via solution process with optimized molar ratio exhibit good electrical properties. a-ITZO films fabricated via spin-coating are a visible alternative to those fabricated via high-cost sputtering methods, and are applicable in flexible and transparent electronics.